In recent years, in the field of sounds, a development or popularization of a system for recording, transmitting, and reproducing space information from the entire circumference has advanced. In a super high vision, for example, broadcasting has been programmed in a three-dimensional multi-channel acoustics of 22.2 channel.
Further, also in the field of virtual reality, a system that reproduces, in addition to a video surrounding the entire circumference, a signal surrounding the entire circumference with respect to sounds is getting out into the world.
An expression method regarding three-dimensional audio information that is able to flexibly respond to an arbitrary recording and reproducing system, which is called ambisonics in the above field, is used and noticed. In particular, the ambisonics in which an order is a second order or higher is called a higher order ambisonics (HOA) (for example, refer to NPL 1).
In the three-dimensional multi-channel acoustics, sound information spreads over a space axis in addition to a time axis. A frequency transformation is performed regarding an angular direction of three-dimensional polar coordinates in the ambisonics, that is, a spherical harmonic function transformation is performed to hold information. Further, when considering only a horizontal plane, an annular harmonic function transformation is performed. The spherical harmonic function transformation or the annular harmonic function transformation can be considered to correspond to a time frequency transformation to the time axis of an audio signal.
An effect of the above method lies in the fact that it is possible to encode and decode information from an arbitrary microphone array to an arbitrary speaker array without limiting the number of microphones or speakers.
On the one hand, as a factor in a hindrance to spreading the ambisonics, a speaker array including a large amount of speakers is required for a reproduction environment or a range (sweet spot) in which a sound space is reproducible is narrow.
For example, to raise a spatial resolution of sound, a speaker array including more speakers is required. However, it is unrealistic that such a system is built at home or the like. Further, an area capable of reproducing a sound space is narrow in a space as in a movie theater and it is difficult to give a desired effect to all spectators.